In prior-art motion indicating devices of this type, rotatably mounted rollers, wheels, or round disks are used as scanning members, and they lie, by their own weight and/or additionally spring-tensioned, in a frictionally engaged manner, on the flat surfaces of the moving objects, whose movement is to be monitored, and they are set into rotation by the moving objects at a circumferential velocity corresponding to the conveyor speed. The signal transmitters, which preferably operate in a contactless manner, may be inductive or consist of optoelectric units, e.g., light barriers. However, these prior-art scanning members require a fixed-position axis of rotation, which must be aligned, as accurately as possible, at right angles to the direction of movement of the objects in order for the scanning member not to be able to cause any change in direction. Consequently, their functionally correct installation is complicated and time-consuming. In addition, special adjusting devices, which entail additional costs, are necessary for this.
Paper jams, which may be caused by, e.g., nonuniform running of the machine, disturbances during folding, and/or damaged paper, may occur in paper-processing machines for sheets of paper, e.g., in folding machines, inserters, offset printers, or in paper-feeding devices.
When additional sheets of paper are introduced into the paper-guiding, -driving, and/or -processing rollers, these may be blocked, so that the entire roller mechanism of such a machine must be opened in order to remove these sheets. Such a paper jam may lead to a prolonged shutdown of the machine, unless the trouble is recognized in time.
The same problems occur in other machines as well, in which material webs are moved. These may be textile materials or even plastics in the form of webs or individual pieces, which are moved along a roller path or a conveyor belt.